This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No.P2001-174291, filed on Jun. 8, 2001; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a switching power supply device capable of setting the number of ringing to a constant number generated during OFF period of switching elements in order to prevent any generation of magneto striction noise from a transformer.
2. Description of the Related Art
FIG. 4 is a diagram showing an example of a circuit configuration of a conventional switching power supply device.
In FIG. 4, the rectification smoothing circuit 11 inputs an alternating current (AC) supply voltage, performing a transmission-wave rectification for it through a diode bridge, for example, and smoothing it through a capacitor, and finally outputs a direct current (DC) voltage obtained to a terminal of the primary winding L1 in the transformer 13.
Other terminal of the primary winding L1 of the transformer 13 is connected to the drain of the switching element Q1. The source of the switching element Q1 is connected to the ground that is also connected to the ground portion of the rectification smoothing circuit 11. A capacitor C1 is connected in parallel between the source and the drain of the switching element Q1.
Magnetic energy accumulated in the primary winding L1 of the transformer 13 is sequentially induced in a secondary winding thereof by switching operation of the switching element Q1 controlled by ON-OFF operation of the control section 25 that will be described later. The half-wave rectification is then performed for the magnetic energy induced at the secondary winding by the diode D1 connected to one terminal of the secondary winding L2, and the smoothing is performed by the capacitor C3, and the smoothed DC voltage is then output to the load 17 and also output to the output DC voltage detection circuit 19.
The output DC voltage detection circuit 19 converts the output DC voltage supplied to the load 17 to a feedback signal and outputs it to the ON-period control circuit 29 mounted in the control section 25.
The diode D3 in the output smoothing circuit 21 performs the half-wave performs the half-wave rectification for a flyback voltage generated at an auxiliary winding L3 in the transformer 13 and the capacitor C5 smoothes the voltage obtained from the diode D3. The control section 25 inputs the smoothed voltage Vcc from the capacitor C5.
The control section 25 initiates oscillation when a starting voltage that is over a predetermined voltage is supplied to the starting resistance R1.
When the switching element Q1 is in OFF, ringing is generated at the primary winding L1 in the transformer 13. The resonance frequency f is as follows:
f=1/(2xcfx80{square root over (Lxc3x97C1))}.
At the same timing, the ringing is also generated at the auxiliary winding L3.
The ringing generation circuit 23 divides the ringing by the resistances R3 and R5 after the detection through the diode D5. The ringing signal whose high frequency components has been eliminated through the resistance R3 and the capacitor C7 is output to the comparator circuit 27 mounted in the control section 25.
The comparator circuit 27 compares the ringing signal input to the comparator COMP1 with a reference voltage Vref1, and outputs a High-level signal when the ringing signal is larger than the reference voltage Vref1.
The ON-period control circuit 29 generates an ON-period control signal to stabilize the output DC voltage to be supplied to the load 17 by adjusting the ON-period according to the feedback signal from the output DC voltage detection circuit 19 and outputs the generated one to the frequency control circuit 31.
The frequency control circuit 31 oscillates a fixed frequency determined by a capacitor and a time constant of a resistance, for example, while controlling the time length of the ON-period according to the ON-period control signal from the ON-period: control circuit 29 and outputs the control signal to the driving circuit 33.
The inverter INV1 in the driving circuit 33 outputs the driving signal V19 of High level to the switching element Q1 when both the control signal V3 from the comparator circuit 27 to one terminal of the OR gate OR1 and the control signal V18 from the frequency control circuit 31 are in Low level simultaneously.
Next, a description will be given of the explanation of the basic operation of the conventional switching power supply device with reference to the timing chart shown in FIG. 5.
(1) When an AV voltage is supplied to the rectification smoothing circuit 11, the control signal V18 is output to the OR gate OR1 at the timing to when the starting voltage which is over a predetermined voltage is supplied to the terminal Vcc through the starting resistance R1. At this time, because no voltage (V2) is generated in the auxiliary winding L3 in the transformer 13, the comparator circuit 27 outputs the control signal V3 of Low level.
As a result, the switching element Q1 enters ON state when the gate of the switching element Q1 inputs the driving signal V19 of High level from the inverter INV1, and the direct current flows from the terminal 11c of the rectification smoothing circuit 11 to the ground GND through the primary winding L1 of the transformer 13 and the drain and the source of the switching element Q1. During this process, the magneto energy is accumulated into the transformer 13.
(2) At timing ti, the frequency control circuit 31 outputs the control signal V18 of High level to the OR gate OR1.
As a result, the driving signal V19 of High level from the inverter INV1 is switched to Low level, and the driving signal V19 of Low level is transferred to the gate of the switching element Q1. The switching element Q1 is thereby turned OFF and the magneto energy accumulated in the transformer 13 is induced in the secondary winding L2 and the auxiliary winding L3 simultaneously.
(3) At timing t1 to timing t2, the electric energy discharged through the secondary winding L2 in the transformer 13 is rectified by the diode D1 and smoothed by the capacitor C3. The output voltage rectified and smoothed is then supplied to the load.
At this time, a voltage V2 is generated at the auxiliary winding in the transformer 13. Because the input voltage level of the comparator 27 is over the reference voltage Vref1, the comparator 27 outputs the output voltage V3 of High level.
(4) Immediately before the timing t2, the level of the input voltage V2 from the comparator 27 is gradually decreased.
As shown in FIG. 5, the voltage V2 of the auxiliary winding L3 is reached to a bottom level (B) at timing t3 and then increased after the timing t3 and reached to a high level at timing t4 and decreased after timing t4 again. This phenomenon is called to as xe2x80x9cringingxe2x80x9d. The same ringing phenomenon is also generated in the voltage V1 of the primary winding L1.
(5) At timing t4 to timing t5, the ringing phenomenon that has been explained in the section (4) is occurred one time. Because the conventional switching power supply device operates by a fixed frequency, the control signal V18 of Low level is transferred to the OR gate OR1 at timing t5.
At this time, because both the input terminals of the OR gate OR1 input the signals V18 and V3 of Low level when the level of the voltage V2 is decreased by the ringing phenomenon caused in the auxiliary winding L3 in the transformer 13 and the level of the output voltage V3 of the comparator circuit 27 becomes Low level, the gate of the switching element Q1 inputs the driving signal V19 of High level from the inverter INV1. Thereby the switching element Q1 is turned ON.
Thus, in the conventional switching power supply device, the control voltage V2 caused by the ringing phenomenon in the auxiliary winding L3 in the transformer 13 is turned OFF around the bottom level that is not more than the reference voltage Vref1.
As described above, because the conventional switching power supply device operates by a fixed oscillation frequency, the switching element Q1 is turned OFF when the voltage V2 caused by the ringing phenomenon is around the bottom level that is not more than the reference voltage Vref1.
By the way, when both the control signal V3 to be supplied to the OR gate OR1 from the comparator circuit 27 and the control signal V18 from the frequency control circuit 31 in the driving circuit 33 become Low level simultaneously, the inverter INV1 outputs the driving signal V19 of High level to the switching element Q1. Accordingly, when the timing to be switched to Low level in one of the control signal V3 from the comparator circuit 27 and the control signal V18 from the frequency control circuit 31 is delayed, the timing at which the switching element Q1 is turned ON is delayed from the timing determined by the fixed oscillation frequency.
As a result, following two cases happen: The number of occurrences of the ringing becomes N and N+1 during the OFF-period of the switching element Q1 under the condition of the same load and the same input voltage.
There is a drawback that the OFF-period of the switching element Q1 is changed by one ringing period and the magneto striction noise occurs in the transformer according to the change of the OFF-period.
Accordingly, an object of the present invention is, with due consideration to the drawbacks of the conventional technique, to provide a switching power supply device capable of preventing any magneto striction noise from a transformer.
Briefly, the present invention may be summarized as a switching power supply device capable of preventing occurrence of a magneto striction noise. The switching power supply device comprises a switching element connected to a primary winding of a transformer connected in series to a direct current (DC) power supply,, a rectification smoothing circuit for rectifying and smoothing an alternating current (AC) voltage induced in a secondary winding of the transformer, an output voltage detection circuit for detecting an output voltage rectified and smoothed by the rectification smoothing circuit and outputs a voltage detection signal to a primary circuit of the transformer, a control circuit for controlling ON-period of the switching element in order to stabilize the output voltage based on the voltage detection signal from the output voltage detection circuit, and a bottom detection circuit for detecting a bottom of a ringing in a same timing of the generation of the ringing between the capacitor connected in parallel to the switching element and the transformer during OFF-period of the switching element. In the switching power supply device of the present invention, a bottom-number count circuit for counting the number of occurrences of the bottom of the ringing, as a bottom-number, detected by the bottom detection circuit, a bottom-number storage circuit for storing the bottom-number counted by the bottom-number count circuit, a bottom-number comparison circuit for comparing the bottom-number stored in the bottom-number storage circuit with a current bottom-number in order to check whether both are equal, and an OFF-period control circuit for maintaining the OFF-period of the switching element until the current bottom-number is reached to the bottom-number stored in the bottom-number storage circuit according to the comparison result from the bottom-number comparison circuit.
According to the present invention, the switching power supply device further comprises a reset circuit for resetting the bottom-number stored in the bottom-number storage circuit.
In the switching power supply device described above, the reset circuit resets the bottom-number stored in the bottom-number storage circuit in a case where the bottom-number of the ringing counted by the bottom-number count circuit is on the increase or on the decrease.
The above and other features and advantages of this invention and the manner of realizing them will become more apparent, and the invention itself will best be understood, from a study of the following description and attached claims, with reference had to the attached drawings showing some preferable embodiments of the invention.